Buck converters are commonly used in electronics for changing the voltage or polarity of a power supply. They are often used to provide low voltage, high current output power. Buck converters typically employ two electronic switches (typically MOSFETs) in combination with an output inductor. The switches are alternately turned on, thereby providing voltage pulses to the output inductor.
During portions (dead times) of the switching cycle, both switches are off. When both switches are off, the output inductor produces freewheeling current that flows through the integral body diode of one of the switches. Body diode current produces substantial energy loss due to the forward-bias voltage drop across the diode, thereby reducing the energy efficiency of the buck converter. Additionally, reverse recovery loss occurs when the body diode later becomes reverse-biased. In order to improve the efficiency of buck converters, body diode conduction and reverse recovery losses must be reduced.
In order to provide buck converters with small size and weight, and reduced cost, it is best to operate the buck converter at high frequency. However, energy loss from both body diode conduction and reverse recovery increase dramatically with increasing operating frequency. In this way, body diode conduction and reverse recovery tend to limit the maximum operating frequency of buck converters.
Also, state of the art microprocessors and digital electronics require exceptionally low supply voltages at high current. In order to provide such electrical power from a fixed-voltage DC supply, the power supply must have a greater voltage step-down ratio. However, a high voltage step-down ratio is typically accomplished by reducing the duty cycle of the buck switches, which tends to increase switching losses. The switching losses are also increased by high frequency operation.
Additionally, it is noted that conventional buck converters operate the switches in hard switching mode. That is, the switches are conducting current when they are turned off, and they have voltage applied when they are turned on. Hard switching results in large switching loss and reduced reliability of the switches. It would be an advance in the art of buck converter design to provide a buck converter that has soft switching or nearly-soft switching (i.e. zero- or low-voltage at turn-on, and zero- or low-current at turn off).
More generally, it would be an advance in the art of buck converter design to provide a buck converter having reduced switching loss, body diode current loss and soft switching. Such a buck converter could operate with very high efficiency compared to conventional buck converters, and could operate at high frequency. High frequency capability can provide many benefits such as smaller size and weight, and reduced cost.